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Hole-punch clouds

Mysterious holes in clouds, such as this one, have long fascinated the public and, until recently, baffled scientists. New research shows that turboprop or jet aircraft punch these holes, causing narrow bands of rain or snowfall.

Clouds often contain supercooled water, or water droplets that persist in liquid form despite subfreezing temperatures due to a lack of particles around which to freeze and become ice. A disturbance in a cloud, such as a jet, can trigger freezing, according to a study published in the June 2010 Bulletin of the American Meteorological Society.

As air moves over an aircraft’s wings or propeller tips, it expands and cools. This cooled air stimulates the formation of ice particles among the supercooled water droplets, which become denser and eventually fall from the sky as precipitation.

The peculiar holey clouds, called hole-punch or canal clouds, are the result of this phenomenon.

Since the late 1940s, humans have been intentionally “seeding” clouds, or inducing precipitation with aircraft using substances such as dry ice and silver iodide to stimulate ice formation, often to address drought conditions.

According to Andrew Heymsfield, a scientist with National Center for Atmospheric Research (NCAR) in Boulder, CO and lead author of the new study into the phenomenon, “Just by flying an airplane through these clouds, you could produce as much precipitation as with seeding materials along the same path in the cloud.”